The present invention relates to countergravity casting of metals and metal alloys.
U.S. Pat. Nos. 3,863,706 and 3,900,064 describe countergravity casting process and apparatus which permit the melting of reactive metals and alloys under a vacuum, and the subsequent protection of the melted material by the introduction of an inert gas, such as argon, to a melting chamber. A gas permeable mold is positioned in a mold chamber above and separated by a horizontal isolation valve from the melting chamber. The mold chamber is evacuated and then inert gas, such as argon, is subsequently introduced to the mold chamber to the same pressure as the melting chamber, permitting the opening of the horizontal isolation valve between the mold and melting chambers. The gas permeable mold is lowered to immerse a mold fill tube into the melted material. The mold chamber then is re-evacuated to create a pressure differential sufficient to lift the melted material upwardly through the fill tube into the mold.
In spite of the success of the above countergravity casting process, production experience has identified a number of disadvantages which partially offset its advantages. In particular, the molten metal can not be introduced (countergravity cast) into the mold any more rapidly than the inert gas contained within that mold can be evacuated through its gas permeable wall. Most noticeably, when the molten metal rises beyond approximately two thirds of the height of the mold, the available mold wall surface area through which the remaining gas can be evacuated from the mold diminishes to a point where entry of metal into the top portion of the mold slows significantly. In cast parts with very thin walls, one disadvantage has been a tendency for the relatively slowly moving molten metal, which has lost much of its original superheat during the filling process to that point, to solidify prior to completely filling the cast shape. This results in excessively high rates of scrap in cast parts near the top of the mold, adding cost when prorated over the manufacture of acceptable cast parts.
Moreover, in practice of the above process, removal of reactive gasses from the mold chamber followed by their replacement with inert gas limits exposure of the mold itself to a relatively complete vacuum for only a very brief period of time (e.g. a few seconds). When gas permeable casting molds having interstitial spaces or pores are used in practice of the above process, gasses are trapped in the interstitial spaces or pores within the mold wall. Similarly, when preformed ceramic cores are positioned in the mold to create complex internal passages within a casting, they also have internal porosity which can contain entrapped gas. Exposure of the mold to high levels of vacuum for only a few seconds provides time for some, but not all, of these trapped gas molecules to escape. Backfilling with an inert gas basically reverses the process, pushing the trapped molecules back into the porous areas of the ceramic material. When the mold is filled with liquid metal or alloy, thermal expansion creates a secondary mechanism by which the gas is driven from the interstitial spaces or pores. Particularly when relatively thick castings, or castings containing ceramic cores, are produced using the above process, gas bubbles tend to form as a result of this thermal expansion and sometimes result in internal gas defects in the castings that increase rejection rates at x-ray inspection of the castings, and, occasionally, in external defects which are visually rejected, especially in hot isostatically pressed (HIPped) castings.
An object of the present invention is to provide countergravity casting method and apparatus that overcome the above disadvantages.
The present invention provides in one embodiment method and apparatus for countergravity casting metals and metal alloys (hereafter metallic material) that provide for melting of the metallic material in a melting vessel under subambient pressure, evacuation of a gas permeable or impermeable mold to a subambient pressure, and controlled, rapid filling of the mold while it is maintained under the subambient pressure by applying gas pressure locally on the molten metallic material in a sealed space defined by engagement of a mold base and the melting vessel with seal means therebetween. The gas pressure applied locally in the sealed space establishes a differential pressure on the molten metallic material to force it upwardly through the fill tube into the mold, which is maintained under subambient pressure.
Pursuant to one particular embodiment of the invention, a metallic material is melted in the melting vessel in a melting compartment under subambient pressure (e.g. vacuum of 10 microns or less). Concurrently, a preheated mold and fill tube are placed on a mold base outside of a casting compartment and then moved into the casting compartment where a mold bonnet is placed on the mold base about the preheated mold such that a mold clamp on the bonnet clamps the preheated mold within the mold base and bonnet. The mold fill tube extends through the mold base. The casting compartment and the mold are evacuated to subambient pressure (e.g. vacuum of 10 microns or less). The melting vessel then is moved into the casting compartment below the mold base. The mold base/bonnet are lowered to immerse the mold fill tube in the molten metallic material and to engage the mold base and the upper end of the melting vessel with a seal therebetween in such a way as to form a sealed gas pressurizable space between the molten metallic material in the melting vessel and the mold base. The mold base is clamped to the melting vessel. The sealed space then is pressurized with inert gas, such as argon, to establish a differential pressure effective to force the molten metallic material upwardly through the fill tube into the mold, while the mold is maintained under the subambient pressure. At the end of the defined time interval, the gas pressurization in the space over the molten melt surface is terminated and subambient pressure in the sealable space and casting compartment is equalized such that any metallic material remaining liquid within the mold drains back into the melting vessel. The mold base is unclamped from the melting vessel and the mold base/bonnet lifted to disengage from the melting vessel and withdraw the fill tube from the molten metallic material. The melting vessel is returned to the melting compartment, and an isolation valve is closed. The casting compartment can then be returned to ambient pressure and then opened, and the mold bonnet can be unclamped and separated from the mold base. The cast mold residing on the mold base then is removed and replaced with a new mold to be cast to repeat the casting cycle.
The present invention is advantageous in that the mold can be maintained under a continuous relative vacuum (e.g. 10 microns or less) prior to and during filling with the molten metallic material to reduce casting defects due to entrapped gas in the mold wall/core body, in that the mold fill rate is controllable and reproducible by virtue of control of positive gas pressure (e.g. up to 2 atmospheres) locally in the sealed space to improve mold filling and reduce casting defects due to inadequate mold fill out, especially in thin walls of the cast component, and to enable taller molds to be filled, and in that efficient utilization of the metallic material is provided in terms of the ratio of the weight of the component being cast relative to the total metallic material consumed during it""s manufacture.